Sintered products of inorganic non-metallic or metallic powders have been used as electrical insulating materials such as semiconductor substrates, printed circuit boards, electrical insulating parts; high-hardness and high-precision machining materials such as cutting tools, dies, bearings; functional materials such as grain boundary capacitors, humidity sensors; and precise sinter molding materials.
When inorganic non-metallic or metallic powders are sintered to produce a sintered product, it is often the procedure in the art to add a sinter additive to the powders for such a purpose as lowering the sintering temperature and/or improving the physical properties of the sintered products.
We have proposed in Japanese Patent Application No. 68037/1988 (Patent Kokai H 1-242469) that ultrafines of yttrium oxide are added as a sinter additive to aluminum nitride powders to increase the reactivity between the powders and the additive, thereby providing improved sintering characteristics and improved physical properties of the sintered product. This process is one of mixing of two components, i.e., sintering inorganic powders and ultrafines of the sinter additive which are separately prepared. It requires homogeneous mixing of the two components to be sintered. In this case, ultrafines of the additive have an increased tendency to agglomerate. The mixing operation must be performed very carefully while maintaining the two components in highly dispersed state. Thus, the prior process is not satisfactory for the production of the sintered product.